The introduction of the laser demonstrated the recently discovered physical principle of light amplification through stimulated emission of radiation. In this the atoms or molecules of a crystal, gas or other substance, the medium, are bombarded with externally supplied photons, so that many of the atoms within the medium are raised from a lower or ground state to a higher atomic energy level. Those energized atoms are said to be "excited". If a photon whose frequency corresponds to the energy difference between the excited and ground states of the atom strikes an excited atom, the struck atom is stimulated, and falls back to a lower energy state, releasing that increment of energy as light. The struck atom thereby emits a second photon of the same or a proportional frequency.
The emitted photon is in phase with and in the same direction as the striking photon, a stimulated emission. The two photons continue to travel in the medium and may then strike additional excited atoms, stimulating additional photon emissions, all of which are of the same frequency and phase. As that action at the atomic level rapidly grows, the atoms in the medium discharge in a chain-like reaction to produce a burst of coherent radiation that propagates in a straight line. Low levels of applied incoherent light are thus essentially converted by a laser into an intense essentially monochromatic coherent light beam.
Since discovery of that principle, many practical forms of lasers have been developed and applied in various applications in the professions, business, industry and government, ranking the laser as one of the most important scientific innovations of the twentieth century. So numerous and widespread are its applications, that the laser has quickly become known even to the public at large; and with such variety the lasers have come to be separately categorized or classed. Among those classes is found one whose characteristic accuracy conforms to extremely high or tight tolerances, on the order of a few microradians or better, which are referred to as precision lasers. The present invention is for a precision laser, one that is believed more useful than earlier types, due to the superior beam stabilization attained, an achievement that is believed to place the precision laser described in the following text at the head of its class.
In brief, a practical laser includes a gain module, which contains the light pumping source which produces the energizing photons or, as variously termed, light, the laser medium, referred to as an accumulator, which is optically pumped by light from that source to emit photons, and a resonator, containing two mirrors, located at respective ends of the gain module, for reflecting photons issuing from the ends of the gain module back into that gain module. Essentially the quantity of emitted photons increases as the emitted photons are reflected back and forth between the mirrors through the medium striking additional excited atoms within the medium. One of the two mirrors is partially light transmissive. That mirror additionally passes a coherent light beam generated in the laser medium, that travels in a straight line, which defines the beam axis. The resonator and the gain module are both mounted to a thick metal plate, referred to as the laser bench, with the gain module mounted inside the resonator in between the two mirrors.
As recognized, the foregoing only briefly touches upon the theory and structure of the laser as background to the present invention, and the interested reader may refer to the abundant literature for additional details and theory. The present invention also includes those basic elements.
It is important to note that the direction of the laser beam axis is dependent on the orientation of the two mirrors in the resonator. Those mirrors are aligned with the greatest precision in parallel. This is accomplished, for one, by placing the axes of the mirrors precisely in parallel. Precision lasers require those resonator optics to be extremely stable, which maintains a high quality laser beam and accurate beam pointing. That has not been entirely possible heretofore in some of that laser's applications.
One application for precision lasers is in targeting objects, typically a military function. In that application the laser beam is directed to and strikes a target; and the light is reflected from the target back toward the laser. In one application the light beam is reflected back to a receiver, which derives information from the reflected beam that is useful for targeting. In another important targeting application, a laser guided projectile is guided by the reflected light to its target.
Where the targeting laser is being carried on a fast moving vehicle, the laser is subjected to any shock and vibration encountered by the vehicle. As example, if a projectile is launched from that vehicle during laser targeting, the propellant detonation from that launch produces an intense shock, as high as 100 g's. The targeting laser is subjected to that shock. If the vehicle is moving fast through rough terrain, up and down through a series of pot holes in a road or field, that produces shock and vibration, which the vehicle's shock absorbers mitigate, but, as drivers know, may be unable to fully suppress. Unless the laser structure contains means to counteract that shock and vibration, the laser beam's axis may shift ever so slightly. Under such circumstances and with small targets at large distances from the vehicle, the light beam moves off the target.
Like other apparatus, the laser structure is also subjected to temperature changes, as might occur from day to day or even during the course of a day, particularly when the laser is necessarily located near other equipment that generates heat. Changes in temperature cause most materials, particularly metals, to change their dimension and/or shape to some degree, a well known phenomenon. That characteristic of the material is its thermal coefficient of expansion. During manufacture, the laser's two mirrors are aligned with great precision, placing the two mirrors precisely in parallel. But that adjustment is made in a facility in an environment that is at one ambient temperature. The laser may be subjected to different temperatures during transit to the customer. At the customer's facility it will be subject to the day to day variation. One can understand the drastic ambient temperature change to which a laser that is manufactured in Southern California in November is subjected to when it is delivered to a location in Alaska in the following month.
Another important application is an industrial one. Lasers have been adapted to serve as drills, one that drills small holes through metal and other materials. If the temperature changes and results in a shift of the beam axis, the relative location of successively drilled holes will change. For example, if drilling of a piece is not completed before the end of one business day, it may be completed the next morning. However the room temperature the next morning may differ drastically from that at the end of the prior business day.
With ordinary lasers in most applications, the effects of temperature change are considered minor and may be disregarded. Such is not the case for precision lasers. In a precision drilling operation holes of a diameter of two thousandths of an inch in diameter must be drilled perfectly perpendicular through the surface of the drilled material. The resultant holes cannot be elliptical or extend on a non-perpendicular axis. And hole to hole spacing must be accurate to a like measure.
When the laser resonator's optics, the two mirrors, are hard-mounted to a laser bench on opposite ends of the lasing medium, any bench motion is deleteriously transmitted to the optics, degrading the laser's performance. The beam may wobble or shift ever so slightly with the bench motion. Others have recognized that problem and offered solutions, which, until the present invention, were the best available solutions. Conventionally, to minimize those external effects, the optics are mounted to a resonator structure that is very stiff. And the resonator structure in turn is attached to the laser bench through "slides" or "kinematic balls". The latter devices, however, are not resistant to loads or forces encountered in transportation, external machinery or to other shock and vibration sources.
The prior "slide" approach to resolving the temperature and vibration problem in precision lasers is a slip type joint formed by a pin and hole arrangement. In this structure, one end of the laser media assembly, where the totally reflecting mirror is located, is fixedly mounted to the support bed. At the opposite end, where the partially reflecting mirror, the optical coupler, is located, a bracket containing a longitudinally extending pin is fixed to the assembly end. That pin is inserted through a hole in another bracket that is fixed to the support bed. Through the pin, the bracket thus supports the weight of a portion of the laser assembly, which presses downward on the pin.
If the support bed expands due to temperature change, the bracket simply moves longitudinally, slipping along the pin, but not exerting any force or change on the optical coupler. If any longitudinal vibration and shaking occurs, the same action basically occurs. However, where shaking perpendicular to the pin occurs, depending upon the side clearance between the cylindrical walls of the pin and the like walls of the hole, some slight movement is possible. Although this clearance may be small, and the consequent wobbling insignificant for most applications, even such small variations are important to and are eliminated in the present invention. Moreover, static friction is encountered between the pin and the joint. Due to that friction, the elements do not always return to the same precise location when the temperature restores or the wobble ceases. The invention provides even greater stabilization and precision that such slip joint isolation and/or that with kinematic balls.
Accordingly, a principal object of the invention is to provide an improved precision laser, one in which the resonator's two mirrors are maintained in perfect alignment, even in adverse environmental conditions.
A further object of the invention is to improve axial stabilization of laser beams.
A still further object is to provide an improved precision laser that generates a laser light beam that remains on-axis despite changes in temperature and despite ambient shock and vibration impacting the precision laser.
An additional object of the invention is to provide a precision laser whose beam axis remains within ten or twenty micro-radians of the axis to which it is initially set, notwithstanding subsequent intervening changes of temperature, and/or application of shock and vibration.
And an ancillary object of the invention is to improve the precision and reliability of precision targeting and drilling applications.